The present invention relates generally to the control of hydraulic power devices and in particular to a pneumatic control assembly for automatically shutting down the supply of high pressure hydraulic fluid when the hydraulic power device is in a returning stroke so as to enhance the operation safety of the hydraulic device.
Heavy power devices which output great work or are capable of moving heavy objects are commonly used in for example construction sites or steel workshops. Examples of the heavy power devices are rear-dump trucks and hydraulic crane tracks. To obtain a great power output, most of the heavy power devices are operated hydraulically. A hydraulic power system requires a pump to pressurize the hydraulic fluid and thus supply the high pressure hydraulic fluid that is needed in operating the hydraulic power device. The pump may be driven by means of an electrical motor or an engine. The pump has to be turned on before the hydraulic power device is operated or the pump has to maintain continuous operation in order to supply the high pressure hydraulic fluid. The pump has to be stopped once the supply of high pressure hydraulic fluid is uo longer needed and this may be done by means of for example a clutch or the like coupled between the pump and the motor/engine.
In a regular hydraulically operated device, a controller is provided for the operator to control the supply of the high pressure hydraulic fluid and the moving direction of the hydraulic device. Such a controller may be electrically or pneumatically operated. For certain hydraulic power devices, pneumatic power is more readily available for control purpose, such as a rear-dump truck which itself is equipped with an air compressor or similar device. In such a case, a pneumatic control assembly has advantages over the electrically operated controller.
The control assembly of a hydraulic power device usually comprises two parts, one of which controls the supply of the hydraulic fluid and the other controls the moving direction of the hydraulic power device. It often happens that when the operator switches the direction control to the retracting direction to move the hydraulic power device in the returning stroke which in certain cases requires no supply of the hydraulic fluid, the operator inadvertently leaves the power control in the engaged position which makes the pump continuing supplying the hydraulic fluid to the hydraulic power device. In such a case, damage to the hydraulic power device may occur or even worse, the hydraulic power device may be accidentally actuated and thus causing property and live casualty/damage.
To overcome such a problem, devices that couple the power control of a pneumatic control assembly to the direction control, especially in moving the hydraulic power device in the returning stroke, are known, such as U.S. Pat. No. 6,065,497 to the current applicant. The known device, however, comprises a direction control valve that has a complicated structure, increasing costs of manufacturing and maintenance.
Therefore, an object of the present invention is to provide a pneumatic control assembly comprising a direction control valve having a simple structure.
Another object of the present invention is to provide a pneumatic control assembly comprising a direction control valve having low costs of manufacturing and maintenance.
In accordance with the present invention, there is provided a pneumatic control assembly comprising a simplified direction control valve. The direction control valve comprises a body forming a bore and inlet and outlet ports in fluid communication with the bore. The bore has a throat openably closed by a spring-biased, first blockage plug. An inner rod is movable in the bore with a lower end engageable with the first blockage plug. The inner rod is spring biased to separate from the first blockage plug. An intermediate rod forms a channel having an opening defined in a lower end of the intermediate rod confronting a second blockage plug mounted to an upper end of the inner rod. The intermediate rod is spring biased to separate from the second blockage plug. A vent hole is defined in an upper end of the intermediate rod and in fluid communication with the channel. An expanded piston is formed around the intermediate rod between the opening and vent hole. An outer rod is supported by a biasing spring retained on the piston. When the outer rod is forced toward the intermediate and inner rods, the biasing spring thereof is compressed, driving the intermediate rod toward the inner rod. The opening of the intermediate rod is closed by the second blockage plug. The engagement drives the inner rod to move with the intermediate rod and urges the first blockage plug to open the throat. Working fluid is thus allowed to flow from the inlet port, through the bore, toward the outlet port. When the outer rod is released, the first blockage plug is returned by its biasing spring to block the throat. The opening of the intermediate rod is separated from the second blockage plug to allow the working fluid residual in the bore to vent through the vent hole.